Electronic band structure

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Every solid material has its own characteristic electronic band structure. According to this structure, the electrical behaviour of solids can be explained. The wide range of electrical characteristics observed in various materials is due to variation in band structure. Depending upon the energy difference between conduction and valence band i.e. width of forbidden energy band, solids are classified into three categories:

• Insulators:

The valence electrons are tightly bound with the nucleus and free electrons are totally absent in an insulator. Very high energy is needed to seperate them from the nucleus. Insulators have completely filled valence bands. Conduction bands are completely empty. The energy gap is very large (up to 15eV). When an electric field is applied, some of the electrons jump to the conduction band and become poor conductors. Examples of insulators include mica, diamond and quartz.

• Semiconductors:

In semiconductors, an electron is loosely bound to the nucleus, hence less energy is needed to separate it from the nucleus. The electrical sensitivity of a semiconductor lies between that of an insulator and a conductor. In semiconductors, the valence band is completely filled and conduction band is completely empty. The band gap is comparitively smaller (up to 2eV). Therefore, a smaller amount of energy is required to push the electrons from the valence band to the conduction band, thus making the semiconductor behave like a conductor.

• Conductors

Conductors have free or conduction electrons at room temperature. These electrons are free to move and hence take part in conduction. In conductors, the valence band overlaps the conduction band and there is no energy band gap. Thus a large number of electrons are available. Conductors can have partially filled conduction bands. Free electrons contribute to the flow of current; holes are not created in the valence band.